Waterproofing of basements and other below ground structures

5.4
Also see:
3.1
3.2
BS 8102
Concrete basements. Guidance on the design and construction of in-situ concrete basement structures

5.4.7Waterproofing systems

The waterproofing shall be suitable for intended use and installed in accordance with the design. Items to be taken into account include:

  1. Type A waterproofing barriers
  2. Type B structure, integral
  3. Type C drained cavity
  4. ancillary components.

Appropriate sequencing of work will enable logical and timely construction of the waterproofing system and prevent unnecessary damage to completed elements of work. Installation should be undertaken in accordance with the design and the installation method statement detailing the sequence of works.

Type A waterproofing barrier

Type A systems generally accepted by NHBC when assessed in accordance with Technical Requirement R3 include:

  • Post applied membrane (hot or cold adhesive)
  • liquid-applied membranes
  • geosynthetic (bentonite) clay liners
  • mastic asphalt to BS 6925 or BS EN 12970
  • cementitious systems
  • pre-applied fully bonded systems
  • proprietary systems or products assessed in accordance with Technical Requirement R3.

Plain polyethylene sheet should not be used as a waterproofing system.

Only fully bonded systems assessed in accordance with Technical Requirement R3 for the specific purpose should be used internally or in sandwich constructions.

Design at junctions and corners should account for proprietary components and be in accordance with the manufacturer’s recommendations. Waterproofing barriers should return at corners to prevent water tracking behind.

The substrate to which the Type A system is to be applied should be clean, free from debris and prepared in accordance with the manufacturer’s recommendations. Bonded sheet membranes should only be applied directly to masonry substrates that are smooth and have flush pointed joints.

Type A waterproofing should be installed in accordance with the manufacturer’s instructions by operatives:

  • who are suitably qualified or have been trained by the manufacturer or supplier, and
  • who are fully aware of the design and the manufacturer’s recommendations for installation.

Completed waterproofing should be protected by:

  • protection board, or
  • carefully placed backfill material.

The manufacturer’s recommendations for climatic conditions at the time of installation should be followed.

Type B structure, integral construction, concrete and application

Structural design should be undertaken by an engineer in accordance with Technical Requirement R5. The design of in-situ Type B concrete systems should be in accordance with:

  • BS EN 1992-1-1
  • BS EN 1992-3
  • Chapter 3.1 ‘Concrete and its reinforcement’.

Type B systems acceptable to NHBC include:

  • in-situ concrete with or without admixtures and crack widths limited by design
  • in-situ high-strength concrete with crack widths limited by design and post-construction crack injections
  • precast concrete systems assessed in accordance with Technical Requirement R3.

Specialist advice should be sought where other Type B systems are specified. BS 8102 contains guidance for the use of Type B systems, including secant, contiguous and sheet piles.

Ready-mixed concrete should be of sufficient strength and durability, and from a supplier operating under a quality control system acceptable to NHBC such as:

  • the Quality Scheme for Ready-Mixed Concrete (QSRMC), or
  • the BSI Kitemark scheme.

Other suppliers may be accepted if they operate to a standard acceptable to NHBC.

The concrete mix should be agreed between the engineer and the waterproofing design specialist, and:

  • achieve the necessary robustness, durability and waterproofing
  • be suitable for the environmental exposure and ground conditions.

Type B waterproofing should be installed:

  • by suitably qualified operatives who are fully aware of the requirements for placing concrete and reinforcement and for installing ancillary components used in Type B systems
  • in accordance with the design.

The line, level and position of formwork and reinforcement should be checked prior to concrete placement to ensure that it is in accordance with the design.

Penetrations from tie bars etc. should be made good in accordance with the design.

Where joints are formed in concrete, surfaces should be clean and free of excessive laitance. Hydrophilic strips should be protected from water before the joint is formed.

Quality management systems and quality audits should be used to record and monitor the placement of concrete on site. Monitoring records should be supplied to NHBC as requested.

Design details for reinforced concrete structures should include:

  • Concrete specification.
  • The type of concrete.
  • Concrete strength.
  • Proportion of any admixture.
  • Proposals for limiting crack widths.
  • Consideration of temporary support to the formwork.
  • Type and position of reinforcement.
  • The method of making good holes in the concrete formed for shutter bolts and tie bars.
  • Positioning of structural elements.
  • Appropriate tolerances for the line and level of structural elements.

Joints between components, including day work joints, should be durable and made watertight with appropriate waterstops or hydrophilic strips. Kickers, generally cast as part of the slab, should be used to form the joint between floors and walls.

Concrete with admixtures

Where the design of in-situ concrete waterproofing includes admixtures:

  • the ratio of admixture to concrete specified in the design should take account of the recommendations of the admixture supplier
  • the reinforcement should be used to control crack widths, which should be in accordance with the design, but not be greater than 0.3mm max. for flexural cracks and 0.2mm max. for cracks that pass through the section
  • suitable quality management systems and quality audits should be used to record and monitor the batching of admixture.

Admixtures should be:

  • independently assessed, in accordance with Technical Requirement R3
  • assessed according to the intended use
  • used strictly in accordance with the manufacturer’s recommendations.

Concrete without admixtures

Where the design of in-situ concrete waterproofing does not include admixtures:

  • high-strength concrete may be specified in order to achieve the necessary level of waterproofing, but post-construction crack injection may be required in order to deal with cracking induced by increased thermal and shrinkage strains
  • the reinforcement should be used to control crack widths, which should be in accordance with the design, but not be greater than 0.2mm max. for both flexural cracks and for cracks that pass through the section
  • a minimum section thickness of 250mm should be used in the design.

Type C drained cavity construction

Type C systems that include a cavity drain membrane which forms a waterproof barrier are acceptable to NHBC when assessed in accordance with Technical Requirement R3. Where a Type C system is formed using a drained masonry cavity wall, the guidance in BS 8102 should be considered.

Type C systems should be designed to include a drainage system that adequately disposes of water to a suitable outlet, either by gravity or through a sump and pump. The drainage channel, sump and pump should include appropriately located access points for servicing and maintenance. To prevent backflow, the drainage system should be fitted with a one-way valve.

Type C waterproofing should be installed in accordance with the manufacturer’s instructions by operatives:

  • who are suitably qualified or have been trained by the manufacturer or supplier
  • who are fully aware of the design and the manufacturer’s recommendations for installation
  • using the fixings recommended by the manufacturer.

Pump systems should operate automatically and include:

  • a primary pump
  • a secondary pump with battery or generator backup
  • a suitable audio or visual alarm that indicates pump failure.

Ancillary components

Ancillary components should be assessed as part of the waterproofing system. Alternatively, an assessment of compatibility and satisfactory performance should be provided for materials and products that are interchangeable between different systems.

Ancillary components include:

  • preformed junctions and corners
  • reinforcement
  • waterstops
  • hydrophilic strips.